Since SNN insisted, I guess I'll go back to testing more levels for the sake of giving some feedback.
I'll have to agree with aj here, this level took a while for me to beat without savestates. If I had to point to anything in particular that's making this level so difficult, it's the overall lack of powerups, particular in the first half where I found the level to be most hazardous.
This is assuming the player doesn't kill the first Frostee in the level (which I can see a lot of players might ignore him). If the player accidentally goes too far left after hitting the first ON/OFF (I'm talking next to the solidfied lavafall), the Frostee will appear and can follow you down here, making it quite tricky to get past the water unharmed.
Course I never liked that particular Frostee to begin with. I can see most players entering this level for the first time jumping too high towards the upper waterfall and getting smacked by him before they can get away.
This spot was a bit annoying. Consider that this is a steep slope that pushes you towards the Thwomp. Note that the Ice Bro can also hit me where I was positioned left of the Thwomp in this screenshot. Also consider that he throws in upward arcs, so even spin-jumping the Thwomp could be a bad idea.
This was a case of requiring a few things to line up correctly to not get hurt here.
Is it really necessary to put spikes next to one of the scarce few powerups like this?
These two powerups are located so close to each other and a Midway Point that they end up being quite moot, considering Midway Points are essentially a free mushroom as it is.
Keep in mind that up until near the midway point, there are only two powerups that are actually helping the player. This isn't enough, considering how easily one can get hit and lose those powerups throughout the first half.
The second half of the level wasn't as difficult as the first in terms of design, but there were absolutely no powerups until you reached one of the exits. I would recommend adding a powerup or two somewhere in this stretch.
Like aj said, that second line-ride was pretty ridiculous. I didn't even finish this part without savestates (partly because I just didn't have the time to really put up with repeated deaths this time around), and I found it quite insulting that you can miss the SMWCoin entirely for not having the switch OFF at the end. Anyone who goes through that hell should be entitled to that prize! If you're going to make the player dodge random Bullet Bills, at least remove a few of the Venus Fire Traps, or do away with the bullet generator instead.
It has been mentioned before Frostees and slopes do not mix well.